The present invention relates to infrared analyzers which can be used in a variety of industrial and commercial applications. In particular the present invention is a method and an apparatus for enlarging the spectral range of an acousto-optic tunable filter by extending the operation of the transducer structure to its third harmonic.
It has been known to utilize plural acoustic transducers in an acousto-optic tunable filter system in order to increase the spectral range of the system. For example, U.S. Pat. No. 3,759,603 teaches a system with a plurality of transducers, each of which is operable in consecutive frequency ranges to increase the system pass band. U.S. Pat. No. 3,665,204 teaches an acousto-optic device which utilizes a contribution to light output from the second order harmonic. Finally, U.S. Pat. No. 3,807,799 offers a general teaching of utilizing higher order spectra to contribute to total light output through the provision of additional slit openings in an output slit diaphragm for an acousto-optic device.
It is therefore an object of this invention to provide an acousto-optic tunable filter in which the range is extended by utilizing the third harmonic frequency band of the transducer structure.
It is also an object of this invention to provide a transducer structure in which the capacity may be electronically switched so that the electrical impedance match is held approximately constant over the extended frequency range.
It is still another object of this invention to provide a method of operating an acousto-optic tunable filter with an extended spectral range so that the filter efficiency at the third harmonic band is generally equivalent to the filter efficiency at the fundamental band.